This invention relates to managing the water flow within a fuel cell, and more particularly, managing the differential pressure across a water transport plate to insure desired water flow across the water transport plate.
Water transport plates are porous separator plates used on both the cathode and anode side of an electrode in a fuel cell. By selecting proper pore sizes within the porous plate, liquid transport occurs but gas transport is prevented. Liquid transport permits membrane hydration and enables removal of product water on the cathode side resulting from the electrochemical reaction within the fuel cell. Preventing gas transport inhibits the fuel and oxidant gases from commingling.
The water transport plates are exposed to water flow fields to maintain desired operation of the fuel cell. In local regions of the cell in which the reactant gas is flowing from a region of low temperature to higher temperature, water evaporates off the porous plate to saturate the gas stream with water vapor; in regions where the reactant gas is moving from higher temperature to lower temperature, product water that is formed in the electrochemical reaction and liquid water that condenses out of the cooling gas stream must be recaptured by the porous plate. In either case, there must be a driving force to move the water through the pores of the water transport plate from the gas flow field side to the water flow field side to remove product water.
Many fuel cell systems use external circulating water cooling loops that add weight and are subject to freezing in cold weather conditions. In some configurations, the external water cooling loop is connected to a diaphragm and is used to provide the driving force to move water across the water transport plate. An air inlet pressure also acts on the diaphragms. It is desirable to limit the amount of water used in a fuel cell to reduce concerns relating to draining, pumping, freezing and thawing the water. As a result, there has been an effort to eliminate the external water cooling loop. What is needed is a driving force across the water transport plate without using pumps or complex valves in a fuel cell without an external water cooling loop.